This invention relates to cameras.
The invention is especially concerned with the dynamic range of such cameras.
TV cameras such as TV broadcast cameras, CCTV, domestic video cameras, are provided with various servo mechanisms to enable satisfactory video signals to be produced from the imaging device, usually a CCD sensor, from a wide range of scene illuminations. The most common servo mechanism is an auto-iris, and in fact these can cope with a large dynamic range, particularly if a transparent plate with an attenuating region or regions in the iris plane, denoted a spotted iris, are employed. The tendency has however been for the dynamic range between different scene illuminations (inter-scene) to be dealt with by electronic shuttering. This means that in scenes of lower illumination, the CCD or other sensor is exposed for a full field of 20 milliseconds (ms) but with higher illumination the imager is gated so as to be exposed for perhaps only 2 microseconds (μs).
Variations of illumination within a scene (intra-scene) present more problems. In a scene in which part is brightly illuminated and part is in shadow, if the region of shadow is of interest, then, if this signal is to occupy the full dynamic range of the camera then the signals corresponding to the brightly illuminated region would greatly exceed the dynamic range of the camera, and therefore must either be clipped at peak level, thereby discarding the detail in the bright region, or compressed, thereby greatly reducing the details in the dark region.
A multi-exposure technique has been demonstrated to overcome this problem (e.g. WO-A-94/18801). CCD sensors may be gated to take two exposures of the same scene with different sensitivities e.g. 100:1 ratio. For example, alternate fields could be gated at 20 ms and 200 μs. Combining these two fields produces a compressed picture. This technique is used for example when broadcasting sports events in which the action moves between one part of the ground in full sunlight and another part in shadow.
A drawback however is that the vertical resolution and temporal resolution are reduced with this technique.
Intra-scene dynamic ranges are of course decreased if the CCD sensor is used in conjunction with an image intensifier (ICCD), since the noise level is now increased. In this case, imaging of scenes illuminated by starlight would be possible. Such an ICCD sensor has been disclosed in our patent application no. GB-A-2 318 012, in which coloured stripes were placed in front of the image intensifier, and processing electronics were used, to enable a monochrome sensor to produce a colour image.
If the double gating technique was extended to ICCDs as well as CCDs, there would be limitations about how fast the two exposures could be alternated. For example, if it was decided to expose twice within a field to reduce loss of vertical resolution, this might not be possible because of phosphor lag in the image intensifier. This is because an image intensifier has a photocathode on which an image is focussed, means such as a micro-channel plate to multiply the number of electrons generated at the photocathode, and an anode on which the electrons impinge. The anode is coated with a phosphor to generate an optical image which is then picked up by a CCD. There is a limitation as to how fast the intensity of illumination of the phosphor can change.
The problem of double exposure with intensifiers could be solved by an arrangement of two intensifiers and a beam splitter passing, say, 90% intensity to one intensifier and 10% to the other. However, the cost and bulk of the camera would then increase, and the intensifier receiving the greater intensity of illumination would require replacement at more frequent intervals than the other.